An image density of a toner image printed on a belt varies depending on circumstances such as temperature or humidity inside or outside an apparatus. The image density of a toner image indicates a value obtained by dividing the gross weight of toner on a region by the area of the region. An image forming apparatus maintains image quality such that the image density does not vary.
The image forming apparatus patterns a toner image for density adjustment on the belt. The image forming apparatus detects an attached toner amount of the toner image using an optical sensor. The image forming apparatus detects the image density of the toner image based on the attached toner amount.
The sensor outputs different voltages according to a toner amount based on an amount of reflected light. The image forming apparatus obtains the attached toner amount based on the sensor output voltage.
The sensor has input and output characteristics. A transverse axis in the sensor characteristics denotes an attached toner amount. A longitudinal axis denotes a sensor output voltage. In a curve indicating the characteristics, the sensor output voltage decreases downward to the right according to an increase in the attached toner amount.
However, a controller cannot read the sensor output voltage with high accuracy in a range where the attached toner amount is large on the curve.
On the curve, a variation amount of a value read by the sensor is great at a side where the attached toner amount is smaller. The variation amount of a value read by the sensor is very small at a side where the attached toner amount is larger.
The image forming apparatus connects an A/D (analog to digital) converter to an output side of the sensor. The A/D converter A/D converts the variation amount, for example, in a quantization step of 1/256.
A variation amount of a value read by the sensor is very small at a side where the image density is higher, that is, the attached toner amount is larger.
The image forming apparatus is difficult to analyze an output from the A/D converter. The image forming apparatus cannot obtain a value read by the sensor with high accuracy at a side where the image density is higher.